Many geothermal sources currently under investigation or exploitation produce large quantities of hot brine at moderate pressures, typically about 150 psia. Some sources, however, produce fluid mixtures of steam and brine at much higher pressures, for example, 800 psia., or higher. In the latter cases, the brine is usually very corrosive giving rise to problems in its use and disposal. Recently, geothermal wells, e.g., in Hawaii, have been drilled producing high pressure geothermal fluid that is about 80% steam and 20% brine. The steam is usually only saturated, and there is some question as to whether these wells will maintain their present pressure in the face of continuous use over the years.
To take this uncertainty to account, it has been conventional to install pressure reducing valves into the flow from the well so that a low pressure steam system can be used in the expectation that the high pressure eventually will fall. However, this is a conservative design, and it is costly over the life of the plant, because a considerable amount of potential power will have been lost.
A back pressure steam turbine driving a generator would seem to be an alternative approach in that the high pressure steam from the well could be converted to lower pressure steam by the turbine and applied in parallel to a plurality of modules that can operate on low pressure steam. Each module may utilize a low pressure steam turbo-generator, and a condenser that acts as a vaporizer for an organic vapor turbo-generator. When the geothermal fluid produces high pressure, saturated steam, expansion of the steam in a turbine can take place in the wet region of the temperature-entropy diagram, producing exhaust steam that contains water droplets and is thus not suitable for application to the inlet stages of low pressure steam turbines in the various modules.
An improved approach is disclosed in copending patent application Ser. No. 07/955,454 filed Oct. 2, 1992 (the disclosure of which is hereby incorporated by reference). In this approach, a geothermal power plant operating on high pressure geothermal fluid includes a primary separator for separating the geothermal fluid into two channels, one containing high pressure steam and the other containing high pressure liquid. The high pressure steam expands in a primary steam turbine (called a back pressure steam turbine) for generating electricity and producing expanded high pressure steam. A secondary separator separates the expanded high pressure steam into an expanded steam component and a liquid component. A primary heat exchanger is responsive to the high pressure liquid for transferring heat to the expanded steam component thereby producing low pressure steam and cooled high pressure liquid.
At least one power plant module is provided which includes a low pressure steam turbine responsive to the low pressure steam for producing electricity and expanded low pressure steam; a condenser/vaporizer containing an organic fluid for receiving the expanded low pressure steam and converting it into condensate, and for vaporizing the organic fluid; an organic vapor turbine responsive to vaporized organic fluid produced by the condenser/vaporizer for generating electricity and for producing expanded organic fluid; a condenser for condensing the heat depleted organic vapor into a liquid; a preheater for heating said liquid; a pump for returning heated liquid from said preheater to said condenser/vaporizer; and a conduit for directing condensate from said condenser to said preheater.
While this approach precludes the application of wet steam to the steam turbines in the various modules of the power plant downstream of the back pressure turbine, it does not address another serious problem that arises because of the high pressure of the geothermal steam. This problem is leakage of geothermal steam through the seals of the back pressure steam turbine. Ordinarily, steam leakage is not a serious environmental problem; but geothermal steam, and particularly high pressure geothermal steam, usually contains a sizable percentage of noxious non-condensable gases such as hydrogen sulfide, and carbon dioxide, whose release into the ambient atmosphere causes serious environmental problems. As the steam pressure increases, the problem of seal leakage increases, and becomes so serious that continued plant operation can be jeopardized.
It is therefore an object of the present invention to provide a new and improved method of and apparatus for generating power from high pressure gases, and fluids as well, and capable of operating on high pressure geothermal fluid without substantially any significant release of noxious gases into the atmosphere.